The most commonly used multi-beam electron guns employed in color CRTs direct three inline electron beams on the inner surface of the CRT's faceplate. A magnetic deflection yoke disposed outside of the CRT's glass envelope sweeps the three electron beams in unison across the faceplate in a raster-like manner. The three electron beams are aligned generally horizontally, or in the direction of each sweep across the CRT's faceplate.
Over the past several years, design of high resolution color CRT electron guns has evolved from the individual type of main lens design to the common lens type design. In the individual type main lens design, inside each of the three guns (red, blue, green) the electron beam goes through an individually defined lens space without sharing this space with the other beams. While this type of electron gun is simple and straightforward, it suffers from the limitation that each gun has a very limited space, resulting in high spherical aberration and generally poor electron beam spot resolution at high beam current.
The so-called "common lens" design has a single, shared aperture for the three electron beams. Each of the three beams goes through its own individual beam path, plus a shared focusing region. The common lens design dramatically reduces spherical aberration in the horizontal direction and also somewhat reduces spherical aberration in the vertical direction. Referring to FIG. 1, there is shown a longitudinal sectional view of a prior art color CRT 20. CRT 20 includes a three beam inline electron gun 21 having three cathodes 32, 34 and 36 for generating three groups of energetic electrons and directing the electrons through three apertures in a G.sub.1 control electrode 38, or grid as these charged elements are sometimes referred to. Electron gun 21 further includes a G.sub.2 screen electrode 40 which similarly includes three inline apertures, each aligned with a respective aperture in the G.sub.1 control electrode 38. The G.sub.1 control electrode 38, the G.sub.2 screen electrode 40 and a facing portion of a G.sub.3 electrode 42 define a beam forming region (BFR) of electron gun 21. Electron gun 21 further includes a high voltage focusing region comprised of the G.sub.3 electrode 42 and a G.sub.4 electrode 44. The three inline apertures of each of the aforementioned electrodes are aligned with a respective one of the cathodes 32, 34 and 36 so as to define three center axes 50, 52 and 54. A convergence cup 46 is attached to the high side of the G.sub.4 electrode 44 for supporting the electron gun 21 within the neck portion of the CRT's glass envelope 22 and for connecting the G.sub.4 electrode 44 to an anode voltage source (not shown) by means of a conductive film 30 disposed on the inner surface of the funnel portion of the glass envelope. The three electron beams are swept in unison across the inner surface of the CRT's display screen 24 by means of a magnetic deflection yoke 48. Disposed on the inner surface of the display screen 24 is a phosphor layer 26 which emits the three primary colors of red, green and blue when the three electron beams are incident thereon. A charged shadow mask 28 disposed adjacent to the CRT's display screen 24 and including a large number of apertures for passing the electron beams serves as a color selection electrode in permitting each electron beam to be incident upon selected areas of the phosphor layer 26.
A second generation of electron guns incorporating a common lens employs an auxiliary asymmetric lens within a focusing electrode to correct for asymmetric electron beam spots 62 on the CRT's display screen as shown in FIG. 2. This auxiliary asymmetric lens typically employs non-circular beam passing apertures which are shaped to correct for the asymmetric lens effect and more particularly to provide beam asymmetric aberration and defocusing correction and a more nearly circular electron beam spot on the CRT's display screen. However, the non-circular shape of the auxiliary lens beam passing apertures renders it more difficult to align the various electrodes during electron gun assembly.
The prior art includes various common lens and auxiliary asymmetric lens combinations which correct for beam defocusing and astigmatism in a multi-beam inline electron gun. One such approach is disclosed in U.S. Pat. No. 5,146,133, issued Sep. 8, 1992, employing facing common lens portions in the horizontally elongated G.sub.3 and G.sub.4 electrodes 42 and 44 as shown in the partially cutaway perspective view of FIG. 3. The facing common apertures of the G.sub.3 and G.sub.4 electrodes permit the electric field of the opposed electrodes to penetrate well into the plate electrodes 70 and 72. The lens converging force in the horizontal direction is thus weaker than that in the vertical direction resulting in electron beam astigmatism. In order to correct for astigmatism, the auxiliary aperture is formed in a non-circular shape with the aperture diameter in the horizontal direction smaller than that in the vertical direction. This is shown in FIG. 4 which is an end-on view of the G.sub.4 electrode 44. The G.sub.4 electrode 44 includes a cylindrical electrode having opposed arcuate portions 68a with radii R.sub.1 and opposed upper and lower straight line portions 68b. The points corresponding to the center axes of the three inline cathodes of electron gun are represented as points O, P and Q in FIG. 4, which respectively lie along vertical lines 74, 76 and 78. The G.sub.4 electrode 44 further includes the aforementioned generally flat plate electrode 72 disposed within the cylindrical electrode 68 and including a vertically elongated aperture 75. Respective ends of the plate electrode 72 are provided with curvilinear edges 72a. In assembling the electron gun, a core bar jig 80 (also known as an electrode support rod or mandrel) is inserted between the electrode's horizontally elongated electrode 68 and the plate electrode 72 as shown in FIG. 5. The core bar jig 80 receives the straight line portion 68b of the G.sub.4 electrode 44 during assembly of the electron gun to ensure proper electrode alignment and electron gun concentricity. From FIG. 5, it can be seen that the core bar jig 80 has a non-round outer circumference which distinguishes it from the round beam passing apertures in the remaining portions of the G.sub.3 and G.sub.4 electrodes. The non-circular cross sectional shape of the core bar jig 80 renders it more costly to produce and more difficult to precisely align the electrodes during electron gun assembly.
The present invention addresses the aforementioned limitations of the prior art by providing a main lens design having common apertures in facing relation in two adjacent electrodes of an electron gun for passing three inline electron beams. The main lens electrodes each further include three inline auxiliary beam passing apertures each shaped to facilitate electron gun assembly using conventional round mandrel beading techniques while providing beam asymmetric aberration and defocusing correction.